1. Field of the Invention
The present invention relates to a glass substrate for magnetic disks having high strength, on which a magnetic layer is easily formed by sputtering, and a process for its production.
2. Discussion of Background
As a substrate for magnetic disks to be used for e.g. magnetic disk memory devices, an aluminum alloy substrate has been mainly employed. However, along with the demand for high density recording, a glass substrate has now been employed which is excellent in flatness and smoothness, of which the base material itself is hard and has a high rigidity as compared with an aluminum alloy substrate, and which can be made thin.
For production of a magnetic disk, an under layer treatment is applied to the surface of a doughnut-type substrate, then a magnetic layer is formed by sputtering, a protective layer is formed on the magnetic layer, and a lubricant layer is further formed on the protective layer. When the magnetic layer is formed by sputtering, usually the doughnut-type substrate is put in a sputtering apparatus while it is supported at three points on its outer periphery by metal support members, and the magnetic layer is formed by sputtering while applying a bias voltage to between the substrate and the target.
In a case of a conventionally used aluminum alloy substrate, the substrate itself has electrical conductivity, and thus the substrate is grounded by means of the metal support members, whereby there is no problem with the application of a bias at the time of the sputtering, whereas in a case of a glass substrate, the glass substrate is an insulating material, whereby the substrate cannot be grounded, and thus it tends to be difficult to apply a bias at the time of the sputtering. Further, in the case of the glass substrate which is an insulating material, static electricity is likely to be generated, and attachment of dust is likely to occur.
Further, the glass substrate which is a brittle material, is likely to break during handling or during use, which is regarded as one of the problems. One of factors governing the mechanical strength of a doughnut-type glass substrate for magnetic disks, is scratches which are present on the inner peripheral edge surface of the glass substrate where the maximum tensile stress will be exerted during use of the magnetic disks. As a glass substrate to solve the problem of scratches, JP-A-2-301017 discloses a glass substrate for information recording disks, wherein a continuous layer of an oxide or a continuous layer composed mainly of an oxide having a thickness of from 0.2 to 50 μm, is formed on the inner peripheral side surface or on the inner peripheral side surface and on the surface portion along the inner periphery.